While oncogenic viruses are the initiator of a complex chain of events leading to malignancy, the host cell also exerts control over the expression of transformation. In the case of SV40 or polyoma, these controls determine, at an early stage, the type of interaction the virus will have with the infected cells: lytic or transforming. At a later stage, the host cell can still exert control over the expression of transformation by suppressing the phenotypic effect of the virus. The elucidation of the host cell factors which are important in the expression of transformation and the problem of how they interact with viral functions represent the objective of this research. We are planning to study the factors which determine the stable association between the viral genome and the cellular genome in polyoma-transformed cells, mainly by taking advantage of the situation found in polyoma-transformed rat cells, where the viral DNA is generally present both in an integrated and non-integrated state. We will also study the regulation of the synthesis of T-antigen and viral m-RNA in SV40-transformed cells. Expression of these viral functions appears to be controlled by the position of the transformed cell in the cell cycle. BIBLIOGRAPHIC REFERENCES: Toniolo, D. and Basilico, C. SV40-Transformed cells with temperature-dependent serum requirements. Cell 4, 255 (1975). Burstin, S.J., and Basilico, C. Transformation by polyoma virus alters the expression of a cell mutation affecting cycle traverse. Proc. Nat. Acad. Sci. U.S.A. 72, 2540 (1975).